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PubMed2025

Cell fate specification modes shape transcriptome evolution in the highly conserved spiral cleavage.

Early animal development can be remarkably variable, influenced by lineage-specific reproductive strategies and adaptations. Yet, early embryogenesis is also strikingly conserved in certain groups, such as Spiralia. In this clade, a shared cleavage program (i.e., spiral cleavage) and similar cell lineages are ancestral to at least seven phyla. Why early development is so conserved in specific groups and plastic in others is not fully understood. Here, we investigated two annelid species (Owenia fusiformis and Capitella teleta) with spiral cleavage but different modes of specifying their primary progenitor cells. By generating high-resolution transcriptomic time courses from the oocyte to gastrulation, we demonstrate that transcriptional dynamics differ markedly between these species during spiral cleavage and instead reflect their distinct timings of embryonic organiser specification. However, the end of cleavage and gastrulation exhibit high transcriptomic similarity, when orthologous transcription factors share gene expression domains, suggesting this period is a previously overlooked mid-developmental transition in annelid embryogenesis. Together, our data reveal hidden transcriptomic plasticity during spiral cleavage, indicating an evolutionary decoupling of morphological and transcriptomic conservation during early embryogenesis.

Animals↗
PubMed1998

Dynamic changes of the microtubule system corresponding to the unequal and spiral cleavage modes in the embryo of the zebra mussel, Dreissena polymorpha (Mollusca, Bivalvia).

Unequal cleavage requires a highly organised cytoskeleton. We investigated the localisation of both tubulins and microtubular arrays in Dreissena eggs during and after fertilisation using confocal laser scanning microscopy. Freshly spawned eggs are arrested in metaphase I. A maternal pool of gamma-tubulin is found mainly in the centre of the asters of the meiotic spindle. The paternal pool of gamma-tubulin, present in the fertilising sperm, could not be traced within the egg, but a microtubule-organising centre forms near the male pronucleus at anaphase II. Male and female pronuclei grow as they migrate in the wake of their aster and rendezvous. First cleavage is unequal and starts without pronuclear fusion. At metaphase the two equal-sized asters span the entire egg in a symmetrical arrangement. At late metaphase the spindle shifts along its longitudinal axis into an eccentric position and the peripheral aster takes on an umbrella-like appearance, whereas the central aster remains spherical. The cleavage furrow becomes determined in the circumferential overlap of the asters. The inequality at second cleavage, however, is due to the unequal size of the asters. The third cleavage spindle also has asymmetrical asters and spindle shift was only observed in the D-cell. The spiral character is a result of an asymmetrical organisation of the larger, vegetal aster. Our results show that the arrangement of the gamma-tubulin clusters and of microtubules changes and develops during early development of Dreissena in a way that can explain the axis-generating asymmetries in cell pattern and the spiral sense of cleavage. The major cytological characters expected to direct pattern formation in this phase of development are: size, position, and symmetry or asymmetry of both spindle and asters.

Animals↗
PubMed2000

The unique developmental program of the acoel flatworm, Neochildia fusca.

Acoel embryos exhibit a unique form of development that some investigators argue is related to that found in polyclad turbellarians and coelomate spiralians, which display typical quartet spiral cleavage. We generated the first cell-lineage fate map for an acoel flatworm, Neochildia fusca, using modern intracellular lineage tracers to assess the degree of similarity between these distinct developmental programs. N. fusca develops via a "duet" cleavage pattern in which second cleavage occurs in a leiotropically oblique plane relative to the animal-vegetal axis. At the four-cell stage, the plane of first cleavage corresponds to the plane of bilateral symmetry. All remaining cleavages are symmetrical across the sagittal plane. No ectomesoderm is formed; the first three micromere duets generate only ectodermal derivatives. Endomesoderm, including the complex assemblage of circular, longitudinal, and oblique muscle fibers, as well as the peripheral and central parenchyma, is generated by both third duet macromeres. The cleavage pattern, fate map, and origins of mesoderm in N. fusca share little similarity to that exhibited by other spiralians, including the Platyhelminthes (e.g., polyclad turbellarians). These findings are considered in light of the possible evolutionary origins of the acoel duet cleavage program versus the more typical quartet spiral cleavage program. Finally, an understanding of the cell-lineage fate map allows us to interpret the results of earlier cell deletion studies examining the specification of cell fates within these embryos and reveals the existence of cell-cell inductive interactions in these embryos.

Animals↗
PubMed2002

Embryonic development in the primitive bilaterian Neochildia fusca: normal morphogenesis and isolation of POU genes Brn-1 and Brn-3.

Neochildia fusca is a member of the taxon Acoela, a group of flatworms that, according to some recent molecular phylogenetic analyses, are distinct from other flatworms and constitute a basal branch with a sister taxon relationship to the rest of the Bilateria. In this paper, we analyze early neural development in this species and report the sequence and expression of two Pit-Oct-Unc (POU) genes, NeocBrn-1 and NeocBrn-3. Homologs of these highly conserved genes play a role in neural fate determination in vertebrates, Drosophila and Caenorhabditis elegans. Acoels, including Neochildia, have a unique invariant pattern of early cleavage called duet spiral cleavage. In subsequent cell divisions descendants of the first three micromere duets form an outer layer of epidermal and neural progenitors surrounding the meso/endoderm progenitors, which are themselves descended from the macromere duet 4A, B and the micromere duet 4a, b. Organ formation begins at mid-embryonic stages with the epidermal primordium adopting a ciliated epithelial shape. Sub-epidermally, a bilaterally symmetric brain primordium can be seen at the anterior pole. Laterally and posteriorly, myoblasts form a thin layer underneath the epidermis. In late embryos and juveniles of Neochildia, the brain is formed by a 3-4 cell-diameter-thick layer of neurons forming a cortex surrounding a neuropile that is relatively free of cell bodies. A highly regular "orthogonal" array of muscle fibers penetrates the brain. We have isolated and partially sequenced homologs of the vertebrate Brn-1 and Brn-3 genes, which we call NeocBrn-1 and NeocBrn-3, respectively. These sequences contain and span portions of the POU-specific domain and a homeodomain, and are sequence similar to their respective homologs in vertebrates and Drosophila. RT-PCR reveals that NeocBrn-1 and NeocBrn-3 are expressed from mid-embryonic to adult stages. Whole-mount in situ hybridization shows expression of both genes in distinct subsets of nerve cells in juvenile and adult worms. NeocBrn-1 also appears in a subset of intra-epidermal gland cells. These observations are an initial step towards reconstructing the neural development of a key group of bilaterians, the Acoela. These flatworms, by virtue of their distinct morphology, development and phylogenetically basal placement, are likely to provide key insights into the interpretation of the evolution of metazoan neural architecture.

Amino Acid Sequence↗
PubMed2004

The cell lineage of the polyplacophoran, Chaetopleura apiculata: variation in the spiralian program and implications for molluscan evolution.

Polyplacophorans, or chitons, are an important group of molluscs, which are argued to have retained many plesiomorphic features of the molluscan body plan. Polyplacophoran trochophore larvae posses several features that are distinctly different from those of their sister trochozoan taxa, including modifications of the ciliated prototrochal cells, the postrochal position of the larval eyes or ocelli, epidermal calcareous spicules, and a collection of serially reiterated epidermal shell plates. Despite these differences, chitons demonstrate a canonical pattern of equal spiral cleavage shared by other spiralian phyla that permits the identification of homologous cells across this animal clade. Cell lineage analysis using intracellular labeling on one chiton species, Chaetopleura apiculata, shows that the ocelli are generated from different lineal precursors (second-quartet micromeres: 2a, 2c) compared to those in all other spiralians studied to date (first-quartet micromeres: 1a, 1c). This situation implies that significant changes have also occurred in terms of the inductive interactions that control eye development in the spiralians. Although radical departures from the spiralian developmental program are seen in some molluscs (i.e., cephalopods), the findings presented here indicate that important changes can occur even within the highly constrained framework of the spiral cleavage program. Among spiralians, variation has been reported for the origin of the anterior, sensory, apical organ, which arises from the 1c and 1d micromeres in C. apiculata. The prototroch of C. apiculata consists of two to three irregular rows of ciliated cells but arise from 1q and 2q daughters, similar to that of Ischnochiton rissoi, as well as the gastropod, Patella vulgata. Despite certain early claims, there is no supporting evidence that any of the shell plates arise pretrochally in C. apiculata. The first seven of eight definitive shell plates that arise in the larva originate from shell secreting grooves in the postrochal region (derived from 2c, 2d, 3d). Earlier descriptions indicate that the eighth plate arises later at metamorphosis, and as this is formed posteriorly, it too forms in the postrochal region. On the other hand, epidermal spicules originate from both pretrochal and postrochal cells (1a,1d, 2a, 2c, 3c, 3d). The significance of these observations is discussed in light of various hypotheses concerning the origin of the conchiferan shell. This study reveals conservation, as well as evolutionary novelty, in the assignment of specific cell fates in the spiralians.

Animals↗
PubMed1997

Cleavage patterns and the topology of the metazoan tree of life.

Several major alliances of metazoan phyla have been identified by small subunit rRNA sequence comparisons. It is possible to arrange the phyla to produce a parsimonious distribution of cleavage types, requiring only one change from a radial ancestral condition to spiral cleavage and one other to "idiosyncratic" cleavage; this arrangement is consistent with most of the recent molecular phylogenies. The cleavage shifts are correlated with changes in many of the features that once were used to distinguish Protostomia and Deuterostomia. It is hypothesized that changes in cleavage direction are causally associated with changes in blastomere fates and thus that cleavage type correlates with such features as the identity of mesoderm founder cells, which in turn can constrain the mode of origination of the eucelom. Cleavage changes may also affect the timing of cell fate specification. In a tree that emphasizes cleavage parsimony, radial cleavage, regulative development, and enterocely are ancestral within the Bilateria, and spiral or idiosyncratic cleavages, mosaic development, and schizocely are associated with a change in cleavage direction. Deuterostomy is presumably ancestral and is correlated with radial cleavage for this reason, rather than mechanistically.

Animals↗
PubMed2004

Body handedness is directed by genetically determined cytoskeletal dynamics in the early embryo.

Although substantial progress has been made recently in understanding the establishment of left-right asymmetry in several organisms, little is known about the initial step for any embryo. In gastropods, left-right body handedness is determined by an unknown maternally inherited single gene or genes at closely linked loci and is associated with the sense of spiral cleavage in early embryos. Contrary to what has been believed, we show that temporal and spatial cytoskeletal dynamics for the left- and right-handed snails within a species are not mirror images of each other. Thus, during the third cleavage of Lymnaea stagnalis, helical spindle inclination (SI) and spiral blastomere deformation (SD) are observed only in the dominant dextral embryos at metaphase-anaphase, whereas in the recessive sinistral embryos, helicity emerges during the furrow ingression. Actin depolymerization agents altered both cleavages to neutral. Further, we found a strong genetic linkage between the handedness-specific cytoskeletal organization and the organismal handedness, using backcrossed F4 congenic animals that inherit only 1/16 of dextral strain-derived genome either with or without the dextrality-determining gene(s). Physa acuta, a sinistral-only gastropod, exhibits substantial SD and SI levotropically. Thus, cytoskeletal dynamics have a crucial role in determination of body handedness with further molecular, cellular, and evolutionary implications.

Actins↗
PubMed2005

Trochophora larvae: cell-lineages, ciliary bands and body regions. 2. Other groups and general discussion.

The embryology of sipunculans, entoprocts, nemertines, platyhelminths (excluding acoelomorphs), rotifers, ectoprocts, phoronids, brachiopods, echinoderms and enteropneusts is reviewed with special emphasis on cell-lineage and differentiation of ectodermal structures. A group Spiralia comprising the four first-mentioned phyla plus annelids and molluscs seems well defined through the presence of spiral cleavage with early blastomere specification, prototroch with characteristic cell-lineage, cerebral ganglia developing from cells of the first micromere quartet (i.e., the episphere) and a ventral nervous system developing from the hyposphere. The planktotrophic trochophore was probably the larval type of the ancestor of this group. Another group comprising phoronids, brachiopods, echinoderms and enteropneusts appears equally well delimited. It is characterized by radial cleavage with late blastomere specification, possibly by the presence of a neotroch consisting of monociliate cells, by the absence of cerebral ganglia and of a well-defined brain and paired longitudinal nerve cords developing in connection with the blastopore, and by coelomic organization. Its ancestral larval type was probably a dipleurula. Several characters link rotifers with the spiralians, although they do not show the spiral pattern in the cleavage. Ectoprocts are still a problematic group, but some characters indicate spiralian affinities.

Animals↗
PubMed1976

Cell differentiation during early development of Nassarius reticulatus L. (Gastropoda, Prosobranchia). II. Morphological changes of nuclei and nucleoi.

In Nassarius reticulatus the nuclei and nucleoli undergo important morphological changes from the zygote to the 16-cell stage. In the zygote and in the trefoil (2-cell) and 4-cell stage, several agranular, fibrillar nucleolus-like bodies 1 mum diameter are present in the interphase nucleus. Granular nucleoli first appear at the 8-cell stage. These nucleoli have fibrillar and granular regions. The granular regions are made up predominantly of ribosome-liek osmiophilic granules. From the 16- and 32-cell stage onwards, the one or two spherical nucleoli of each nucleus measure 2.5 mum in diameter and show a concentric organization with a very dense central region surrounded by a broad peripheral zone containing numerous granules, possibly of ribonucleoprotein. At the same time the number of ribosome-like particles increase in the karyoplasm and that of ribosomes in the cytoplasm. These findings are surprising because in eggs with radial cleavage, which have been subjected to more detailed analysis, the first granular nucleoli appear after the end of the cleavage, at the blastula or gastrula stage. The early appearance of granular nucleoli which seem to be characteristic of several eggs with spiral cleavage is discussed in connection with biochemical data on RNA-synthesis.

Animals↗
PubMed2003

The MAPK cascade in equally cleaving spiralian embryos.

Spiralian development is shared by several protostome phyla and characterized by regularities in early cleavage, fate map, and larva. Experimental evidence from multiple spiralian species implicates cells in the D quadrant lineage as the organizer of future axial development of the embryo. However, the mechanisms by which the D quadrant is specified differ between species with equal and unequal spiral cleavage. Equally cleaving mollusc embryos establish the D quadrant via cell-cell interactions between the micromeres and macromeres at the 24- to 36-cell stage. In unequally cleaving embryos, the D quadrant is established at the 4-cell stage via asymmetries in the first 2 cell divisions. We have begun to explore the molecular mechanisms of D quadrant patterning in spiralians. Previously, we showed that, in the unequally cleaving embryo of the mollusc Ilyanassa obsoleta, the MAPK pathway is activated and functionally required in 3D and also in the micromeres known to require a signal from 3D. Here, we examine the role of MAPK signaling in 4 spiralians with equal cleavage. In 3 equally cleaving molluscs, the chiton Chaetopleura, the limpet Tectura, and the snail Lymnaea, the MAPK pathway is activated in the 3D cell but not in the overlying micromeres. In the equally cleaving embryo of the polychaete annelid Hydroides, MAPK activation was not detected in the 3D macromere but was observed in one of its daughter cells, 4d. In addition, inhibiting Tectura MAPK activation disrupts differentiation of 3D and cells induced by it, supporting a functional role for MAPK in axis specification in equally cleaving spiralians. Thus, MAPK signaling may have a conserved role in the D quadrant organizer cell 3D in molluscs. However, there have been at least 2 evolutionary changes in the activation of the MAPK pathway during spiralian evolution. MAPK function in the Ilyanassa micromeres is a recent cooption and, since the divergence of annelids and molluscs, there has been a shift in onset of MAPK activation between 3D and 4d. We propose that this latter shift correlates with a change in the timing of specification of the secondary embryonic axis.

Animals↗
PubMed2004

Trochophora larvae: cell-lineages, ciliary bands, and body regions. 1. Annelida and Mollusca.

The trochophora concept and the literature on cleavage patterns and differentiation of ectodermal structures in annelids ("polychaetes") and molluscs are reviewed. The early development shows some variation within both phyla, and the cephalopods have a highly modified development. Nevertheless, there are conspicuous similarities between the early development of the two phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia have almost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except for minor variations between species. The cell-lineage of the metatrochs is almost unknown, but the telotroch of annelids and the "telotroch" of the gastropod Patella originate from the 2d-cell, as does the gastrotroch in the few species which have been studied. The segmented annelid body, i.e. the region behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells just in front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the mollusc body is covered by descendants of cells from both the C and D quadrants and a growth zone is not apparent. This supports the notion that the molluscs are not segmented like the annelids, and that the repeated structures seen in polyplacophorans and monoplacophorans do not represent a segmentation homologous to that of the annelids.

Animals↗
PubMed2002

Conserved mechanism of dorsoventral axis determination in equal-cleaving spiralians.

Many members of the spiralian phyla (i.e., annelids, echiurans, vestimentiferans, molluscs, sipunculids, nemerteans, polyclad turbellarians, gnathostomulids, mesozoans) exhibit early, equal cleavage divisions. In the case of the equal-cleaving molluscs, animal-vegetal inductive interactions between the derivatives of the first quartet micromeres and the vegetal macromeres specify which macromere becomes the 3D cell during the interval between fifth and sixth cleavage. The 3D macromere serves as a dorsal organizer and gives rise to the 4d mesentoblast. Even though it has been argued that this situation represents the ancestral condition among the Spiralia, these inductive events have only been documented in equal-cleaving molluscs. Embryos of the nemertean Cerebratulus lacteus also undergo equal, spiral cleavage, and the fate map of these embryos is similar to that of other spiralians. The role of animal first quartet micromeres in the establishment of the dorsal (D) cell quadrant was examined in C. lacteus by removing specific combinations of micromeres at the eight-cell stage. To follow the development of various cell quadrants, one quadrant was labeled with DiI at the four-cell stage, and specific first quartet micromeres were removed from discrete positions relative to the location of the labeled quadrant. The results indicate that the first quartet is required for normal development, as removal of all four micromeres prevented dorsoventral axis formation. In most cases, when either one or two adjacent first quartet micromeres were removed from one side of the embryo, the cell quadrant on the opposite side, with its macromere centered under the greatest number of the remaining animal micromeres, ultimately became the D quadrant. Twins containing duplicated dorsoventral axes were generated by removal of two opposing first quartet micromeres. Thus, any cell quadrant can become the D quadrant, and the dorsoventral axis is established after the eight-cell stage. While it is not yet clear exactly when key inductive interactions take place that establish the D quadrant in C. lacteus, contacts between the progeny of animal micromeres and vegetal macromeres are established during the interval between the fifth and sixth round of cleavage divisions (i.e., 32- to 64-cell stages). These findings argue that this mechanism of cell and axis determination has been conserved among equal-cleaving spiralians.

Animals↗
PubMed2004

Embryonic development of the oligochaete Enchytraeus coronatus: an SEM and histological study of embryogenesis from one-cell stage to hatching.

We describe the embryonic development of the soil-living oligochaete Enchytraeus coronatus (Enchytraeidae, Oligochaeta, Annelida). Enchytraeus coronatus is a direct developer. It follows the typical spiral cleavage mode of development that is highly conserved among annelids and a large number of other lophotrochozoan taxa that are collectively named "Spiralia." Scanning electron microscopy (SEM) was combined with light microscopic analysis of wholemounted and sectioned embryos, differentially processed through histological stainings, to reconstruct and document cellular movements and organogenesis from early cleavage stages until hatching. With the help of these data we have established a scheme of morphologically defined stages in order to facilitate future studies on the molecular and histological level that will allow a detailed cross-species comparison among annelids and other phyla.

Animals↗
PubMed1996

Dual origins of mesoderm in a basal spiralian: cell lineage analyses in the polyclad turbellarian Hoploplana inquilina.

Evolutionary modifications in the origins and organization of the mesoderm represent significant events in the diversification of metazoan body plans. Within the Spiralia, mesoderm comprises ectomesoderm, which typically is generated by derivatives of the second and third quartets of micromeres, and endomesoderm, which is formed from the fourth quartet micromere of the D quadrant (4d). It has been held that endomesoderm generates the majority of adult mesodermal derivatives, while larval mesoderm is formed primarily from ectomesoderm. The evolutionary history of these mesodermal sources could be clarified by examining basal members of the Spiralia such as the polyclad turbellarians, whose embryos exhibit canonical quartet spiral cleavage. Using the fluorescent lineage tracer DiI, we show that larval mesoderm is derived from only two cells, one from the ventral embryonic quadrant (2b, the "mesectoblast" cell), and the other from the dorsal quadrant (4d, the mesentoblast cell). We compare these results with mesodermal origins in other spiralian phyla and conclude that a dual origin of mesoderm is a primitive feature of spiralian development. We also argue that ectomesoderm and endomesoderm should not be considered as the exclusive precursors of larval and adult mesoderm, respectively.

Animals↗
PubMed1998

The cell lineage of a polyclad turbellarian embryo reveals close similarity to coelomate spiralians.

Recent molecular evidence suggests the turbellarian Platyhelminthes may represent the extant basal members of the Spiralia and therefore probably exhibit ancient features of the spiralian developmental program. The stereotypic quartet spiral cleavage pattern of the polyclad turbellarian embryo, among other features, indicates that this group may be closely related to the ancestral flatworm; however, polyclad embryos have been the subject of few experimental studies. Here we report the results of a cell lineage analysis of the embryo of the polyclad Hoploplana inquilina based on microinjection of DiI into cleavage-stage blastomeres following formation of each of the four quartets of micromeres. The first quartet gives rise to most of the lateral and anterior ectoderm of the Müller's larva; the second quartet forms largely dorsal and ventral ectoderm as well as the circular muscles; the third quartet forms only small clones of ectoderm; and only the 4d cell of the fourth quartet contributes to larval structure, forming the longitudinal muscles, mesenchyme, and probably endoderm. Our results demonstrate a striking similarity between the cell lineages of polyclad and higher spiralian embryos, in which the four quadrants also bear the same relationships to the larval axes and give rise to comparable larval structures, including derivation of mesoderm from both ectodermal (2b) and endodermal precursors (4d).

Animals↗
PubMed2000

The embryonic development of the polyclad flatworm Imogine mcgrathi.

In this paper we describe the embryonic development of the polyclad flatworm Imogine mcgrathi. Imogine is an indirect developer that hatches as a planctonic Goette's larva after an embryonic period of approximately 7 days. Light and electron microscopic analyses of sections of staged embryos were combined with antibody stainings of wholemounted embryos to reconstruct the origin and movement of the primordia of the various organ systems, with particular emphasis on the nervous system. We introduce a system of morphologically defined stages aimed at facilitating future studies and cross-species comparisons among flatworm embryos. Imogine embryos undergo typical spiral cleavage. Micromere quartets 1-3 form an irregular double layer of mesenchymal cells that during gastrulation expands over micromere quartet 4. Micromere 4d divides into several large mesendodermal precursors whose position defines the ventral pole of the embryo. These cells, along with the animal micromeres that obtained a sub-surface position during cleavage, form a deep layer of cells that gives rise to all internal structures, including the nervous system, musculature, nephridia, and gut. Micromeres 4a-c are large yolky cells that are incorporated into the lumen of the gut, but do not themselves contribute to the gut epithelium. Shortly after gastrulation, cell differentiation sets in. Cells located at the surface adopt epithelial characteristics and form cilia that result in continuous movement of the post-gastrula stage embryo. Deep cells at the lateral margins of the embryo become organized into a protonephridial tube. A cluster of approximately 50 deep cells at the anterior pole forms the brain, in which we have identified sets of founder neurons of the brain commissure and the dorsal and ventral connectives. The early differentiating neurons, along with other cells forming stabilized microtubules (ciliated cells of the epidermis, gut and protonephridia; apical gland cells) could be analyzed in detail because of their labeling with an antibody against acetylated alpha-tubulin. Our findings indicate that, despite significant differences in the cleavage pattern and arrangement of blastomeres in the early embryo, morphogenesis and organ formation of a polyclad embryo follows a pattern that is very similar to the pattern observed by us and others in phylogenetically more evolved rhabdocoel flatworms.

Animals↗
PubMed1997

The early emergence of platyhelminths is contradicted by the agreement between 18S rRNA and Hox genes data.

The phylogenetic position of the platyhelminths within the metazoan tree is examined using two independent sets of molecular characters, the evolution of 18S ribosomal RNA sequences and the diversity of the genes belonging to the HOX cluster. Among the various hypotheses that have been considered by zoologists, a position of the platyhelminths within the protostomes, related to the phyla with typical spiral cleavage, appears to be favoured when taking into account all separate lines of evidence. It is in conflict with the traditional hypothesis of an early emergence at the base of the bilaterally symmetrical animals. This relatively late emergence is compatible with the old idea that flatworms are derived from a coelomate ancestor. New evidence from the sequences of Hox genes suggests that the duplicated genes Ultrabithorax/abdominal-A constitute a genetic synapomorphy of the whole protostome clade.

Animals↗
PubMedDate not supplied

Animal phylogeny and the ancestry of bilaterians: inferences from morphology and 18S rDNA gene sequences.

Insight into the origin and early evolution of the animal phyla requires an understanding of how animal groups are related to one another. Thus, we set out to explore animal phylogeny by analyzing with maximum parsimony 138 morphological characters from 40 metazoan groups, and 304 18S rDNA sequences, both separately and together. Both types of data agree that arthropods are not closely related to annelids: the former group with nematodes and other molting animals (Ecdysozoa), and the latter group with molluscs and other taxa with spiral cleavage. Furthermore, neither brachiopods nor chaetognaths group with deuterostomes; brachiopods are allied with the molluscs and annelids (Lophotrochozoa), whereas chaetognaths are allied with the ecdysozoans. The major discordance between the two types of data concerns the rooting of the bilaterians, and the bilaterian sister-taxon. Morphology suggests that the root is between deuterostomes and protostomes, with ctenophores the bilaterian sister-group, whereas 18S rDNA suggests that the root is within the Lophotrochozoa with acoel flatworms and gnathostomulids as basal bilaterians, and with cnidarians the bilaterian sister-group. We suggest that this basal position of acoels and gnathostomulids is artifactal because for 1,000 replicate phylogenetic analyses with one random sequence as outgroup, the majority root with an acoel flatworm or gnathostomulid as the basal ingroup lineage. When these problematic taxa are eliminated from the matrix, the combined analysis suggests that the root lies between the deuterostomes and protostomes, and Ctenophora is the bilaterian sister-group. We suggest that because chaetognaths and lophophorates, taxa traditionally allied with deuterostomes, occupy basal positions within their respective protostomian clades, deuterostomy most likely represents a suite of characters plesiomorphic for bilaterians.

Animals↗